The worldwide demand for energy in the 21st century is growing at an alarming rate. TheEuropean ¿World Energy Technology and Climate Policy Outlook¿ [WETO] predicts anaverage growth rate of 1.8% per annum for the period 2000-2030 for the world energydemand (European Commission, 2003). The increased demand is being met largely byreserves of fossil fuel that emit both greenhouse gases and other pollutants. Since the rate offossil fuel consumption is higher than the rate of fossil fuel production by nature, thesereserves are diminishing and they will become increasingly expensive.Against this background, the transition towards a sustainable, carbon-free and reliableenergy system capable of meeting the increasing energy demands becomes imperative.Renewable energy resources, such as wind, solar, water, wave or geothermal, can offer cleanalternatives to fossil fuels. Despite of their obvious advantages renewable energy sourceshave also some drawbacks in their use because they are unevenly distributed both over timeand geographically. Most countries will need to integrate several different energy sourcesand an advanced energy storage system needs to be developed.